An exhaust gas emitted from diesel engines contains PM (particulate matter) based on carbonaceous soot and SOF (soluble organic fraction) of high-boiling-point hydrocarbons. When such exhaust gas is released into the atmosphere, it may adversely affect human beings and the environment. For this reason, a PM-capturing ceramic honeycomb filter, which may be called “honeycomb filter” in short, has been disposed in an exhaust pipe connected to a diesel engine. One example of honeycomb filters for purifying an exhaust gas by removing particulate matter is shown in FIGS. 2(a) and 2(b). The honeycomb filter 10 comprises a ceramic honeycomb structure comprising porous cell walls 2 defining large numbers of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4, and an outer peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c alternately sealing the outlet-side-sealed flow paths 3 and the inlet-side-sealed flow paths 4 on the exhaust-gas-inlet-side end 8 and the exhaust-gas-outlet-side end 9 in a checkerboard pattern. The honeycomb filter is disposed in a metal container (not shown), with its outer peripheral wall 1 gripped by a holding member (not shown) constituted by a metal mesh, a ceramic mat, etc. such that the honeycomb filter used is stationary.
In the honeycomb filter 10, an exhaust gas is cleaned as follows. As shown by the dotted arrow, the exhaust gas flows into the outlet-side-sealed flow paths 3 opening at the exhaust-gas-inlet-side end 8. While it passes through the cell walls 2, specifically through penetrating holes constituted by communicating pores on and in the cell walls 2, PM contained in the exhaust gas is captured. The cleaned exhaust gas is discharged from the inlet-side-sealed flow paths 4 opening at the exhaust-gas-outlet-side end 9 into the atmosphere.
As PM continues to be captured by the cell walls 2, penetrating holes are clogged with PM on and in cell the walls, resulting in increased pressure loss when the exhaust gas passes through the honeycomb filter. Accordingly, it is necessary to burn PM before the pressure loss reaches the predetermined level to regenerate the honeycomb filter. However, when burning a large amount of PM captured, combustion heat causes melting erosion in the honeycomb filter. This melting erosion tends to occur in portions of the honeycomb filter near the exhaust-gas-outlet-side end 9, in which PM is likely accumulated, particularly in center portions of planes perpendicular to the axial direction of the honeycomb filter.
JP 2005-2972 A discloses a honeycomb filter comprising heat-absorbing portions having larger heat capacity than that of other portions in an exhaust-gas-outlet-side portion, so that the combustion heat is absorbed and dissipated. Specifically, the heat-absorbing portions having larger heat capacity to absorb the combustion heat of PM are formed by making the cell walls thicker in the exhaust-gas-outlet-side portion, providing the cell walls with smaller porosity in the exhaust-gas-outlet-side portion, or making the outlet-side plugs longer. However, partial increase in the cell wall thickness or partial decrease in the cell wall porosity necessitates new steps, suffering large cost increase despite insufficient prevention of the melting erosion of the honeycomb filter. Although longer outlet-side plugs surely provide the entire honeycomb filter has higher heat capacity, the combustion heat of PM is not effectively absorbed by the entire outlet-side plugs, failing to sufficiently prevent melting erosion.
JP 2000-279729 A, JP 10-52618 A and WO2004/113252 A disclose technologies of controlling the porosity, pore diameter and surface roughness of cell walls. JP 2000-279729 A describes the relation between an average pore diameter and the surface roughness Rz (10-point average roughness) of the cell walls when the porosity of the cell walls changes; the larger the porosity or pore diameter, the larger the surface roughness of the cell walls. JP 10-52618 A discloses a method for producing a honeycomb structure having a surface roughness (Rz) of 30 μm or more and/or a pore opening diameter of 20 μm or more in the cell walls, comprising the steps of forming a honeycomb molding with a conductive material, supplying electric current to the honeycomb in the axial direction of penetrating holes in a non-oxidizing atmosphere to cause the cell walls to generate heat to complete sintering. It is described that because crystal particles near the surface having larger free energy are predominantly sintered, grain growth in the sintering is used by this method to provide cell walls with rougher surfaces. WO2004/113252 A discloses that extrusion die slits having larger surface roughness Ra can provide cell walls with larger surface roughness Ra. However, it is difficult to apply the technologies of JP 2000-279729 A, JP 10-52618 A and WO2004/113252 A for controlling the porosity and surface roughness of cell walls to plugs.